In order to minimize signal bandwidth requirements, broadcast television uses the interlaced scan line technique. In one scan field the odd lines are displayed (lines 1, 3, 5, etc.) and in the next scan field even lines are displayed (lines 2, 4, 6, etc.). In the US NTSC broadcast standard, scan fields are displayed at a rate of 60 fields per second, with odd and even scan field alternating. As such, odd scan fields are displayed 30 times per second and even fields also 30 times per second. In the PAL broadcast standard, there are 50 fields per second, or 25 odd and even fields per second.
Human visual persistence is such that the 50 or 60 fields per second causes little noticeable image flicker, assuming the adjacent odd/even field lines are similar. In normal broadcast picture content, image processing is imposed such that only subtle pixels value changes occur between adjacent odd and even lines. This means that the differences in the odd and even fields are minor, so essentially the same image is being refreshed at 50 or 60 times per second, minimizing flicker.
Progressive Scan Computer Images
Computer displays use the non-interlaced or progressive scan line technique. The entire image (including each sequential scan line) is displayed 60-90 times per second, depending on the system. In computer applications, interlace scan would cause severe image flicker since there are drastic pixel value changes between adjacent scan lines within the text and graphics images of typical applications. For example, using interlaced scan, a horizontal white line that is one pixel high on a black background would noticeably flicker at 30 times per second (using a 60 frame/sec interlaced display). By using progressive scan, fine vertical detail can be used on the computer display without producing noticeable flicker.
Computer Display on the Broadcast Television
There are products on the market that adapt the progressive scan computer video output to an interlaced scan video format suitable for a broadcast television receiver. The simplest of devices do this conversion with no temporal image processing. This typically results in severe image flicker problems because of the sharp changes in vertical pixel values as explained above. More elaborate devices apply a vertical image filter, commonly known as a "flicker filter", prior to interlacing the image. The flicker filter essentially blends the value of vertically adjacent pixels to decrease the differences in adjacent odd/even lines. This dramatically reduces the noticeable image flicker, but equally as noticeably reduces the level of vertical detail as compared to the original computer display. Thus, the general consensus has formed in the computer industry that high resolution graphics is not compatible with the interlaced display format used in broadcast television.
The Standard 3-Line "Flicker Filter"
The current standard in the PC industry for interlace flicker reduction is the three line flicker filter. The three line flicker filter takes three vertically adjacent pixels from the progressive-scanned source image, blends them using a weighted average, and outputs this value as the new pixel for the interlaced display. In effect, this creates a low-pass vertical spatial filter. This filter 10 is conceptually illustrated in FIG. 1. Conceptually, as is seen, the above pixel 12, the current pixel 14 and below pixel 16, are combined to provide resulting blended pixel 18. The filter is represented functionally in FIG. 2. Functionally, as is seen, the above adjacent pixel value 12 is combined with a first weighting coefficient 22 at combiner 28, current pixel 14 is combined with a second weighting factor 24 at combiner 30, and the below adjacent pixel value is combined with a third weighting coefficient 26 at combiner 32. The output from combiners 28, 30 and 32 are provided to adder to provide the new pixel value 18. The most common pixel weighting used in the filter is shown in Table 1. Using these weighting values, the filter is sometimes referred to as a "1-2-1 Flicker Filter". All flicker filter examples shown in this section use this 1-2-1 or 25%-50%-25% coefficient weighting.
For the purpose of illustration in this section a pixel is treated as a single value of contrast energy in relation to surrounding pixels. In actual systems, each pixel is represented by three discrete values of RGB (Red, Green, and Blue) color components. In standard applications of the 3-line flicker filter each RGB component is processed through a separate 1-2-1 weighted 3-line flicker filter.
Standard Weighting Values for the 3-Line Flicker Filter
To further assist in understanding the effects of the 3-line flicker filter, the center source pixel, having the highest weighting in the output pixel, should be thought as the current scan line pixel. The three-pixel or triad window is effectively scanned across the image a row at a time such that each pixel is passed through the filter's center position. This is illustrated in FIG. 3. In total, each source pixel is passed through the vertical filter 3 times (once in the top position, once in the middle, and once in the bottom position).
As the title of FIG. 3 implies, the image is "effectively scanned" where as in the actual circuit the filter stays fixed and the streaming pixel data is passed through it. In implementation, since three scan lines of pixels are needed the two previous lines are stored which produces a one line processing latency relative to the "current" (center) scan line.
Processing of Color Components
As stated above, the image processing of the flicker filter is typically applied to the RGB components of each pixel. The flicker filter can also be applied to the image data after is converted to YUV component data which is required as part of the composite video encoding process. RGB to YUV conversion is performed with a linear matrix multiplier common called a "color matrix" 40 as shown in FIG. 4. Assuming that no other non-linear processing such as Gamma correction is performed on the video signal prior to the filter, the flicker filter can be applied to either the RGB or YUV data with no difference in outcome.
Flicker Filter Advantage: Flicker Reduction of Horizontal Lines
Perhaps the most compelling reason to use a flicker filter is to remove the bothersome 30 Hz flicker of a single-pixel-high horizontal line in a graphics image or text. Without the flicker filter, the single horizontal line will be displayed in only the even field or odd field of the interlaced image, but not both. Using a 3-line flicker filter, as illustrated in FIG. 5, the contrast energy of the line is spread onto adjacent lines which implies into the other field when interlaced scanned at the output.
The contrast energy of the image line, originally concentrated in a single field, is now evenly distributed between the two interlaced fields. Thus, rather than contrast energy changing between odd and even field, resulting in a noticeable 30 Hz flicker, contrast energy is constant in both fields thereby refreshed at 60 times per second. This virtually eliminates visible image flicker, assuming that viewing distance is such that the image is observed as a whole, not up close for single pixel line inspection (which would be only several inches from monitor).
Image features taller than 1 pixel high are also pixel reduced. As with single pixel lines, this is due to the blending of line values at the feature's edge or contrast transition, effectively increasing refresh rate of the transition. The flicker filter spreads the contrast transition between adjacent lines in odd and even fields. There can still remain a second order effect that is referred to as flitter, however, as discussed in a later section.
Flicker Filter Disadvantage: Loss of Vertical Resolution
Because the 3-line flicker filter is essentially a vertical low-pass filter, flicker reduction is accomplished in the trade for reduced vertical resolution. Reduced vertical resolution can be noted in FIG. 5 for a single pixel horizontal line. FIG. 6 shows the result of different arbitrary pixel pattern.
Thus, the vertical filter attributes of the 3-line flicker filter limits the ability to display fine vertical detail.
Vertical Resolution Loss
From a user application standpoint, the biggest problem of the decreased vertical resolution is in the display of text. Crisp text requires high resolution in both the horizontal and vertical axis. The 3-line flicker filter requires that the text font is made larger to remain easy to read on the interlaced scan TV display, as compared to the progressive scan computer display. FIG. 7 shows examples of text resolution loss following 3-line flicker filter processing.
Regarding FIG. 7, the following assumptions are made regarding the stated lines per frame quantity: The image display is 640.times.480 pixels, underscanned. At 40 lines per frame, the "e" character is 5 pixels tall, whereas a capital letter is 9 pixels tall, with 3 pixels of spacing between lines, At 20 lines per frame, all quantities are doubled; the "e" character is 10 pixels high, capital letters are 18 pixels tall, with 6 pixels spacing between lines.
From FIG. 7, it can be judged that with 3-line flicker filter 20 character lines per frame could be comfortably read while 40 lines per frame could not. Thus while the filter is effective in reducing annoying image flicker, it also limits the usefulness of the interlaced display for images rich in textual content such as web pages, email, word processing, etc.
Summary of the Standard 3-Line Flicker Filter
The 3 line flicker filter is the widely accepted standard in the computer industry due its modest complexity and overall effectiveness. In comparison, more sophisticated approaches are used in the TV broadcast industry to processes computer generated images resulting in both superior image resolution and reduction of flicker. As a result, graphics images processed through a standard 3-Line flicker filter falls short of the expected TV quality established by the broadcast industry, independent of the composite video encoder quality.
Accordingly, what is needed is a method and system for improving the image quality of an interlaced display. The present invention addresses such a need.